How to assign an image to Image Node by its name
you can iterate over the currently referenced images in bpy.data.images.

def get_img_reference(suffix):
    images = bpy.data.images
    for img in images:
        if suffix in img.name:
            return img

img = get_img_reference('_Roughness')
if img:
   image_node.image = img


It's possible to write this shorter but the extra checking is a sanity check. This is case sensitive, beware :)